The invention relates to a method for process monitoring during diecasting or thixotropic moulding of metals. The invention also relates to a metal diecasting or thixotropic moulding device.
In the car industry in particular, increasingly high demands are made on tolerances and on the mechanical properties of diecastings and thixotropic mouldings. In order to meet these high quality demands, the most comprehensive degree of monitoring of the method parameters and their reproducibility are of great importance.
In order to monitor a diecasting or thixotropic moulding process, firstly the condition of the metal introduced into the casting chamber and secondly the parameters of the diecasting or thixotropic moulding process are decisive. In order to optimise the diecasting or thixotropic moulding process and to evaluate all parameters which are critical for process stability and reproducibility, as far as possible all parameters which can affect the process need to be covered.
A key factor for achievement of high reproducibility and process stability is the condition of the thixotropic metal rod and the diecasting alloy on introduction into the casting chamber, whereby the temperature of the thixotropic rod or the diecasting alloy represents a very important parameter.
In order to check and monitor the diecasting or thixotropic moulding process, temperature measurements may be undertaken in the alloy melt and inside the thixotropic metal rod during the heating process, whereby the temperature distribution may for example be determined using thermo-elements at various melt or rod positions (inside the rod and around the rod edge), Habitually the heating curves, i.e. temperature as a function of the heating time, relevant for the individual measuring positions, are determined.
Whereas for monitoring alloy melts for diecasting, in essence temperature measurement is used, measurement of the electrical heating energy applied during preheating constitutes a further option for monitoring the state of the rod during thixotropic moulding.
For monitoring a thixotropic moulding process, metallographic tests to determine the distribution of the liquid proportion can be undertaken on the thixotropic rod, for example by cutting the rod at various longitudinal positions across its longitudinal axis and determining the liquid proportion in the rod cross-section, for example as a function of the distance from the centre of the rod. The aim of such investigations is to optimise the heating curve in such a way that a predetermined liquid proportion is achieved as homogeneously as possible within the entire thixotropic rod in the shortest possible time. Calorimetric measurements can also be performed in order to determine the mean liquid proportion.
With respect to the parameters of the diecasting or thixotropic moulding process, the temperatures of the casting chamber, the sprue channels and the mould cavity are normally measured, and the pressure and humidity in the evacuated mould cavity are ascertained.
The formerly habitual determination of the parameters with respect to the diecasting or thixotropic moulding material and of the diecasting or thixotropic moulding process is complex and unsuitable for monitoring the diecasting or thixotropic moulding processes under production conditions.
The invention seeks to solve the problem of creating a method for monitoring the process during diecasting or thixotropic moulding of metals, with which the manufacture of diecastings or thixotropic mouldings can be reliably monitored under production conditions.
This problem is solved according to the invention in that the temporal development of the moulding pressure p(t) is measured and the time-related speed of the casting piston v(t) is determined, and the energy E(t) supplied by the casting piston as a function of the process time t, and the total energy Etot supplied by the casting piston during the diecasting or thixotropic moulding process, are calculated on the basis of the time-related development of the moulding pressure p(t) and the casting piston speed v(t), and the total energy Etot is used as a parameter for monitoring the diecasting or thixotropic moulding process.
The method according to the invention is especially suitable for diecasting or thixotropic moulding of aluminum alloys or magnesium alloys.
The method according to the invention is especially suitable for horizontal thixotropic moulding installations and horizontal diecasting plants, i.e. devices in which the casting chamber lies horizontal.
The method according to the invention is based on the fact that the total energy supplied through the casting piston represents an extremely relevant checking parameter for the entire diecasting or thixotropic moulding process. The method according to the invention to determine the energy supplied through the casting piston and the use in particular of the total energy value as a parameter for process monitoring is also known as the RTIM process (Real Time Injection Monitoring).
The preheating temperature and the corresponding temperature distribution in the metal rod, and the measure of the energy supplied through the casting piston in the case of thixotropic moulding, are especially relevant, since a specific liquid proportion which remains within a narrow variation range must be observed. For example, in the case of thixotropic moulding, it may be concluded from a high total energy supplied by the casting piston, that the viscosity of the thixotropic material is too low, which can be caused either by a too small liquid proportion or too low shearing forces during the thixotropic moulding process.
The method according to the invention permits better process stability, optimisation of the process parameters, improvement of product quality and a reduction in the reject rate.
The method according to the invention is used with particular preference for thixotropic moulding. Here, it serves in particular to determine the optimum liquid proportion of the thixotropic metal rod under production conditions. The optimum mean liquid proportion in the thixotropic metal rod in this case is 40-55% by weight. If the liquid proportion is too high, the thixoforming of thixotropic material takes place virtually under the same conditions as the diecasting of liquid metal alloys, so that for example the benefit of a low shrinkage of thixotropic material during cooling in the mould cavity is lost, or else the shearing of the oxide skin surrounding the thixotropic rod is made more difficult or impossible. Moreover, the dimensionally stable insertion into the casting chamber of a thixotropic rod with a high liquid proportion is difficult and in most cases is not reproducible.
A further important factor in the case of thixotropic moulding is the homogeneity of the thixotropic condition, i.e. the distribution of the liquid proportion over the length of the rod and the rod cross section, whereby this homogeneity is generally better, the slower the preheating process is undertaken; on the other hand, the shortest possible heating time is desired for economic reasons.
During the inventive activity it was found for thixotropic moulding, that by determining the total energy supplied through the casting piston to the thixotropic material during a thixotropic moulding process, the liquid proportion existing after the preheating and its homogeneity within the thixotropic rod can be indirectly monitored.
The method according to the invention is further suitable in particular for monitoring the preheating furnaces, i.e. by determining the total energy for each charge, i.e. for each complete diecasting or thixotropic moulding process, with thixotropic rods or diecasting material from a specific preheating furnace, it is possible to ascertain and monitor the regularity of this furnace. Furthermore, by determining the total energy for each charge with thixotropic rods or diecasting material from different preheating furnaces, the regularity of the heating power of the furnaces concerned can be compared and monitored.
Determining the total energy for each charge moreover permits the pre-solidification to be checked. Pre-solidification, i.e. premature material solidification, can for example be caused by too low a casting chamber and/or mould temperature, and is undesirable due to the resulting usually poor casting properties. Thanks to the possibility of ascertaining pre-solidification, it is possible indirectly to check the temperatures of the casting chambers and the casting mould. Furthermore, inferences can be drawn indirectly on the design of the mould cavity.
Furthermore, determination of the total energy for each charge also allows investigation of pressure losses during the charge due, for example, to tribological properties, and thus provides information in relation to the friction of the casting piston, the mechanical condition of casting pistons and/or of the casting chamber, piston lubrication and the effect of parting compounds. Consequently, determination of the total energy supplied to the system by the casting piston during a charge also serves to monitor the tribological conditions in relation to the casting piston and the casting chamber.
According to the invention, the process time-dependent speed v(t) of the casting piston can either be directly measured or determined by measuring the process time-dependent casting piston position s(t).
On the basis of the time-dependent position measurement s(t) of the casting piston, the time-dependent speed v(t) of the casting piston can be calculated using the function
v(t)=ds(t)/dt
i.e. by differentiating the time-dependent casting piston position s(t) after time t. The speed of the casting piston on the basis of the position measurement s(t) is suitably calculated at discrete, for example equidistant times. Suitably the speed is calculated at 50 to 800, preferably at 180 to 500 and in particular at 250 to 400 discrete process times. The discrete speed values thus ascertained are preferably filtered using numeric methods. Furthermore, a constant speed curve v(t) is calculated, preferably using numeric interpolation methods.
The time-dependent energy supplied by the casting piston during two process times tx and ty can be calculated according to the integral function
            E              t        ,        y              ⁡          (      t      )        =      A    ·                  ∫                  t          x                          t          y                    ⁢                                    p            ⁡                          (              t              )                                ·                      v            ⁡                          (              t              )                                      ⁢                  ⅆ          t                    
where A is the area of the casting piston facing the continuous casting or thixotropic moulding material.
The energy E(t) supplied by the casting piston, as a function of process time t, can be calculated according to the integral function
      E    ⁡          (      t      )        =      A    ·                  ∫                  t          0                t            ⁢                                    p            ⁡                          (              t              )                                ·                      v            ⁡                          (              t              )                                      ⁢                  ⅆ          t                    
and the total energy Etot supplied through the casting piston during the diecasting or thixotropic moulding process can be calculated by the integral function
      E    tot    =      A    ·                  ∫                  t          0                          t          4                    ⁢                                    p            ⁡                          (              t              )                                ·                      v            ⁡                          (              t              )                                      ⁢                  ⅆ          t                    
where A is the area of the casting piston facing the diecasting or thixotropic moulding material and t0 is the starting time t=0 of the diecasting or thixotrapic moulding process and t4 the time at which the casting piston first assumes the speed v(t)=0 after t0. At time t4, the actual diecasting or thixotropic moulding process is completed and the mould cavity is filled in order to balance out any material shrinkage during cooling of the moulding in the mould cavity and to avoid corresponding incomplete mould filling, the pressure on the diecasting or thixotropic moulding mass is also maintained for a short time after t4, so that the casting piston can make a further translational movement, where the casting piston speed can again fall to zero.
Instead of measuring the time-dependent casting piston position s(t), the time-dependent development of the speed v(t) can be directly measured and used for calculation of the energy E(t) supplied by the casting piston or the total energy Etot.
The piston position s(t) or the piston speed v(t) and the development of the pressure p(t) during the entire diecasting or thixotropic process are preferably continuously measured.
An important feature of the invention is also the fact that the measurements s(t) and p(t), or v(t) and p(t), and the calculation of v(t), E(t) and Etot can be undertaken online during the process, so that the parameters are available immediately after the charge for corresponding correction measures, i.e. the measurement of s(t) or v(t) and p(t), and determination of v(t) and E(t) takes place in real time. A process window is available between two charges, which permits intervention via the undertaking of correction measures, since immediately after the charge the moulding has to be cooled again, the mould opened, the moulding removed from the diecasting or thixotropic moulding device and the casting chamber recharged with the diecasting or thixotropic moulding material. The casting chamber is preferably loaded with a thixotropic metal rod using a robot. The casting chamber is loaded with a liquid metal alloy for diecasting for example by opening a valve or plug in a trough, so that the liquid metal can flow into the casting chamber.
Preferably, in addition to total energy Etot, the total energies E1 to E4 supplied by the casting piston can be determined for the following process stages:
in thixotropic moulding, the partial energy E1 supplied by the casting piston during the period between time t0 and time t1 for moving the thixotropic metal rod in the casting chamber until the rod reaches the end of the casting chamber facing the mould, where t1 is the time at which the metal rod reaches the end of the casting chamber; in a diecasting process, the partial energy E1 is constantly zero;
in diecasting or thixotropic moulding, the partial energy E2 supplied by the casting piston during the period between t1 and time t2 for deforming the thixotropic metal rod or the diecasting material, whereby t2 is the time at which the diecasting or thixotropic moulding material fills the entire casting chamber cross section over its entire length;
in diecasting or thixotropic moulding, the partial energy E3 supplied by the casting piston during the period between t2 and time t3 for filling the sprue channels, where t3 is the time at which the sprue channels between the casting chamber and the mould cavity are all completely filled;
in diecasting or thixotropic moulding, the partial energy E4 supplied by the casting piston during the period between t3 and time t4 for filling the mould cavity, where time t4 is the time at which all parts of the mould cavity are completely filled and the speed of the casting piston has fallen to zero, i.e. v(t4)=0.
In particular, ascertaining the partial energy E1 permits the pre-solidification of the diecasting or thixotropic moulding material in the casting chamber to be determined. Moreover, E1 in particular also provides information about the general tribological conditions, i.e. for example pressure losses due to friction, wear manifestations and lubrication, and thus serves for example to assess the effect of the parting compounds and lubricants, and furthermore provides information on the friction of the casting piston and its lubrication.
Normally, the casting piston of diecasting or thixotropic moulding installations is driven hydraulically. In diecasting or thixotropic moulding installations designed in this way, the time-dependent pressure development p(t) is particularly advantageously determined by simultaneously measuring the time-dependent pressure development pGK(t) at the casting piston surface facing the diecasting or thixotropic moulding material, and by measurement of the pressure development Phyd(t) in the hydraulic liquid, whereby preferably the pressure development pGK(t) is used for calculation of the energy supplied through the casting piston to the diecasting or thixotropic moulding material.
Since for the monitoring of the diecasting or thixotropic moulding process according to the invention and the corresponding process checking, it is not the absolute energy measurements E(t), Etot, E1 to E4 which are important, but in essence the corresponding energy values for various thixotropic rods or diecasting material quantities from the same preheating furnace or from various preheating furnaces are compared, the pressure development phyd(t) can also be used for calculation of the energy values E(t), E1 to E4 and Etot.
phyd(t) describes the total pressure exerted on the casting piston. However, this does not correspond to the pressure exerted on the diecasting or thixotropic moulding material, since the casting piston itself is exposed to a certain friction in the casting chamber.
As a result, the simultaneous measurement of phyd(t) and PGK(t) permits determination of the pressure lose xcex94p=phyd(t)xe2x88x92pGK(t) due to the friction of the casting piston, and therefore allows inferences to be drawn regarding the mechanical condition of the casting chamber and casting piston and the lubrication of the casting piston.
Whereas Etot permits global checking of the entire thixotropic moulding or diecasting process, the partial energy values E1 to E4 provide information on certain process parameters, such as have been described in the case of E1 for example for the tribological conditions explained above or for ascertaining pre-solidification. E2 is suitable for example for obtaining information on the necessary deformation energy and provides, in the case of thixotropic moulding for example, information about the condition of the rod, i.e. whether the thixotropic rod is too hard or too soft, or whether the liquid proportion is too high or too low. E3 and E4 on the other hand are suitable for example for monitoring the filling behaviour of the sprue channels and the mould cavity, and for example provide information on the effect of the parting compound and in the case of thixotropic moulding, also on the shearing forces acting on the thixotropic material.
In the case of thixotropic moulding or diecasting processes with RTIM process monitoring according to the invention, a report is preferably printed out for each work shift, which is normally of around 8 hours, where preferably the number of casting or thixotropic mouldings manufactured, i.e. the charge number n, the partial energies E1 to E4 and the total energy Etot are calculated for each charge and shown on the report printout. Also, preferably the mean total energy Etot.m and the standard deviation "sgr"n for all n charges with diecasting or thixotropic moulding material from the same preheating furnace are ascertained and printed out.
The mean total energy Etot.m for a number n of charges with thixotropic moulding or diecasting material from the same furnace K is calculated for example as an arithmetic mean as:       E          tot      ,      m        =            1      n        ⁢                  ∑                  i          =          1                n            ⁢              E                  tot          ,          l                    
The standard deviation may then be calculated as       σ    n    =                    1                  n          -          1                    ⁢                        ∑                      i            =            1                    n                ⁢                              (                                          E                                  tot                  ,                  l                                            -                              E                                  tot                  ,                  m                                                      )                    2                    
Also, preferably the relevant scatter can be calculated according to
xe2x80x83"sgr"rel=100%xc2x7"sgr"n2/Etot.m
From an assessment of the resulting mouldings and the corresponding comparison of the energy values Etot and E1 to E4 and of the mean and standard deviation, it is possible to conclude what energy range is admissible in order to obtain a satisfactory moulding quality. Therefore with respect to the energy values Etot and E1 to E4 a nominal value range for the thixotropic moulding or diecasting process can be determined, which can be used as a parameter for example for a process interruption, a change of preheating furnace, a calibration of the heating power of a preheating furnace, a correction of the casting curve or triggering of a monitoring alarm.
With respect to the device, the present invention is based on the task of providing a thixotropic or diecasting device which permits monitoring of the manufacturing process under production conditions.
According to the invention, this is achieved by a diecasting installation comprising a casting chamber, a casting piston movable in the casting chamber, a mould with at least one mould cavity, and means for simultaneously determining a process time-dependent pressure p(t) and a process-time dependent position s(t) of the casting piston.
A diecasting or thixotropic moulding device according to the invention, in which the measuring devices permit continuous recording of the time-dependent pressure p(t) and continuous position measurement s(t), is especially preferred.
The measuring device to determine the position s(t) can also preferably have a device to measure the time-dependent speed v(t) of the casting piston, whereby the position of the casting piston s(tx) at time tx is determined as       s    ⁡          (              t        x            )        =            ∫              t        =        0                    t        x              ⁢                  v        ⁡                  (          t          )                    ⁢              ⅆ        t            
The device according to the invention is especially suitable for thixotropic moulding or diecasting using the method according to the invention for process monitoring.